Line focus electron gun

ABSTRACT

An electron gun transforms a round beam of electrons into a sheet beam by employing a pair of curved deflecting plates positioned between two spaced anodes of a final unipotential lens of the gun.

United States Patent Inventors Lawrence A. Harris [56] References Cited 2 C Md M UNITED STATES PATENTS orman uns y am ge ass. pp No 17,416 2,613,333 /1952 Bull 313/83 X Filed Mar. 9, 1970 Primary Examiner-Roy Lake Patented Sept. 28, 1971 Assistant ExaminerV. Lafranchi Assignee General Electric Company Attorneys-Paul A. Frank, John F. Ahern, Julius J.

Zaskalicky, Frank L. Neuhauser, Oscar B, Waddell and Joseph B. Forman LINE FOCUS ELECTRON GUN 8 Claims, 3 Drawing Figs.

US. Cl 313/82, 313/80, 313/83, 313/268 Int. Cl H0lj 1/88, ABSTRACT: An electron gun transforms a round beam of HOIj 29/02,H01j 29/46 electrons into a sheet beam by employing a pair of curved Field of Search 313/83, 82, deflecting plates positioned between two spaced anodes of a 84, 85, 87 final unipotential lens of the gun.

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LINE FOCUS ELECTRON GUN Our invention relates to electron guns and in particular to electron guns used in analytical apparatus.

The usual electron gun provides a round beam of electrons which irradiates an object upon which it falls. Frequently it is desirable to have a linear beam of electrons which can sweep as a fine line across a specimen, surface, or other object being examined. For example, in apparatus for studying the surfaces of materials such as, for example, that shown in US. Pat. No. 3,461 ,306V. L. Stout and N. R. Whetten, granted Aug. 12, 1969, and assigned to the assignee of this present invention, where the characteristics of a material are determined by Auger analysis, it is essential that the analyzer axis, the specimen surface and the primary beam be concurrent. To increase the sensitivity of such an instrument, it is desirable to have a high primary current density at the proper spot on the target and deflection of the electron beam is required to optimize the beam position. In most electron guns, the beam is deflected after the beam has left the lens system and has acquired its final velocity. This limits the deflection sensitivity and extends the physical length of the gun. Also, prior electron guns designed to produce sheet beams usually consist of a series of electrodes with long narrow slits, all of which must be accurately aligned with each other.

The usual electron gun for producing a primary electron beam employs a cathode, a pair of grids, and a first anode to provide a round electron beam which subsequently is focused by a unipotential lens formed by the first anode, a succeeding anode, and an intermediate lens electrode. The two anodes and the lens electrode provide a lens action. Such a gun, however, has not provision for deflection of the beam and in order to have the beam strike the right spot on a target, the gun must be accurately positioned mechanically, or an auxiliary deflection system external to the gun must be provided to deflect the beam after the electrons have attained their final velocity.

It is an object of our invention to provide an electron gun which employs a lens electrode which operates at low potentials and upon electrons having low velocity.

It is another object of our invention to provide an electron gun having a lens structure incorporated therein for changing an electron beam from a round configuration to a rectangular shape.

To overcome the problems and shortcomings of the conventional electron gun described above, the present invention has been developed. The invention consists of an electron gun which includes a final unipotential lens having a center electrode in the fonn of a pair of curved deflecting plates which operate at potentials near that of the cathode to establish a nonsymmetrical focusing field and alter the shape of the low velocity electron beam from a substantially circular configuration to one that is substantially rectangular in cross section thus minimizing the problem of alignment with other electrodes in any associated system.

While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, the details of the preferred embodiment of the invention may be more readily ascertained from the following detailed description when read in conjunction with the accompanying drawings in which:

FIG. I is a cross-sectional view of an electron gun embodying our invention;

FIG. 2 is a cross-sectional view of the gun of FIG. 1 taken along the lines 22; and

FIG. 3 illustrates the electron gun of our invention as employcd in apparatus for surface analysis by Auger electron emission.

in the electron gun of FIG. 1, a cathode l, which may comprise a ribbon of tungsten, is supported adjacent an aperture 2 in a first control electrode 3. Both'eathode l and electrode 3 are supported from a ceramic disk 4 and cathode 1 is heated by current supplied over leads 5, 6. Grid 3 has a planar surface and is in opposed parallel relation'with a second planar grid 7 also having a'central aperture 8, grid 7 being supported by a metallic disk 9. Spaced longitudinally from grids 3 and 7 of the electron gun 10 are a pair of planar anodes ll, 12 each having central apertures.

Structurally, gun 10 includes end rings 14, 15 connected together by tie rods 16 which tie rods in turn are surrounded by ceramic tubular rods 17. A plurality of insulating spacers 18-22 surrounds rods 17 and maintain the respective electrodes in fixed spaced relation. Of course, any other techniques for supporting electrodes in spaced relation may be employed rather than the one specifically illustrated and described.

In accordance with our invention, we provide means for changing the configuration of the electron beam which passes through the gun from a circular form as it passes through the aperture in anode 11 to a substantially rectangular cross section as it passes through the aperture in anode 12. This means comprises a pair of curved deflecting plates 24, 25 positioned between anodes ll, 12 and having supporting terminals 16, 27 positioned between spacers 21, 22. As may be seen in FIG. 2 of the drawings, electrodes 24, 25 are elliptical in cross section.

Operating potentials near that of the cathode for grids 3 and 7 are provided by leads 30, 31; high positive potentials for anodes I1 and 12 are supplied by leads 32, 33; and controllable potentials near that of the cathode for electrodes 24, 25 are supplied by leads 34, 35.

In the operation of our electron gun, the circular electron beam which traverses the aperture in anode 11 is modified in cross section and direction by the fields due to the potentials on electrodes 24, 25 and onthe anodes ll, 12. The change in beam cross section may be understood by considering electrodes 24 and 25 to be at the same low potential with respect to the potential on anodes 11, 12. This region of depressed potential acts like a conventional unipotential lens to converge the electron beam, but because of the shape of plates 24, 25, this focusing action is much stronger in one direction than in the other. Thus, in FIG. 2, the strong converging action is in the vertical direction, while the beam is left free to diverge in the horizontal direction so that the beam fans out into a flat shape resembling an ax blade. The nonsymmetrical focusing field established by electrodes 24, 25 alters the shape of the electron beam from a circular cross section as it passes through the apertures in anode 11 to a substantially rectangular cross section as it passes through the aperture in anode 12. If now different potentials are applied to electrodes 24, 25 over leads 34, 35 while maintaining the same average potential, a transverse deflecting field is produced in addition to the focusing field described above. This transverse field deflects the beam at right angles to the edge of the aforementioned ax blade shape. Because the deflecting field is applied in a low potential region, where electrons have low velocity, the deflection sensitivity is high, allowing appreciable deflections in a short electron gun with small deflecting voltages. The focusing is obtained by adjusting the average potential of electrodes 24, 25 while the deflection of the resultant sheet beam is obtained by controlling the difference between the potentials applied to these two electrodes. Such control is obtained by a conventional potentiometer (not shown) or any equivalent control. The extent to which the beam fans out in the unfocused direction may be limited by appropriate aperture stops (not shown).

One application of our improved electron gun structure is shown in FIG. 3 in connection with the irradiation of specimens in an electron probe microanalyzer of the type disclosed in the previously mentioned US. Pat. No. 3,461,306- Virgil L. Stout and Nathan R. Whetten. In FIG. 3, the sample holder 40 is rotatable by means of shaft 41 and supports a plurality of specimens 42 which are to be examined. Electron gun 10 provides a sheet beam'43 of electrons which irradiates a specimen 42 and produces a secondary electrons 44 which ass through a narrow slit 45 to'the input 46 of an analyzer. in such apparatus, it is desirable to have-a'flat or.line focus beam which can be deflected inone direction, the width of the beam and its deflection being controlled by control of the potential applied to electrodes 24,25 of the electron gun.

One of the advantages of our invention is that we obtain control of the form and deflection of an electron beam without increasing the physical length of the gun and without altering the axial symmetric construction with the exception of the modified lens electrodes 24, 25. By modifying the cross section of these electrodes, we control the shaping of the electron beam. Thus, by varying the ellipticity of the lens electrodes 24, 25, the electron beam may be varied in cross section from a round beam to an elongated sheet beam even without introducing deflection at the lens.

While we have shown our invention as applied to an electron probe microanalyzer, it is apparent that it may be used in other types of apparatus which employ one-dimensional scanning, such as, for example, radar displays.

While there has been described what is believed to be a preferred embodiment of the present invention, it should be understood that variations and modifications may occur to those skilled in the art.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An electron gun comprising a cathode, first and second planar anodes spaced from said cathode, each of said anodes having a central aperture, a pair of control electrodes positioned between said cathode and said first anode and cooperating therewith to from a circular electron beam which passes through the aperture in said first anode, and means positioned between said anodes for establishing a nonsymmetrical focusing field so that the beam is substantially rectangular in cross section as it passes through the aperture in said second anode.

2. An electron gun comprising an electron-emitting source, a pair of planar anodes spaced from each other and from said source, a pair of control electrodes positioned between said source a first of said anodes, said anodes having aligned apertures and said control electrodes focusing electrons emitted from said source to form a circular cross section electron beam traversing the aperture in said first anode, and electrode means positioned between said anodes for providing a nonsymmetrical focusing field to change the cross section of the electron beam from a circle as it traverses the aperture in said first anode to substantially a rectangle as it traverses the aperture of the second of said anodes.

3. The gun of claim 2 in which said electrode means comprises a pair of curved deflecting plates positioned between said anodes for establishing a transverse deflecting field in addition to the said nonsymmetrical focusing field.

4. The gun of claim 3 in which said plates are opposed elliptical members.

5. An electron gun comprising four spaced, parallel, circular electrodes having axially aligned apertures, thermionic cathode means positioned adjacent the aperture in a first of said electrodes, said electrodes formed into two pairs, the first pair of said electrodes comprising the said first electrode and the one adjacent thereto being maintained at a low potential and the second pair of electrodes comprising the remaining two electrodes being maintained at a high positive potential to from an electron beam having a circular cross section as it traverses the aperture in the first electrode of said second pair of electrodes, and a pair of lens electrodes positioned between the second pair of electrodes for establishing a nonsymmetrical focusing field to change the cross section of the electron beam to a rectangular shape when it traverses the aperture in the second electrode of said second pair of electrodes and a transverse deflecting field to deflect the beam at right angles to its major dimension.

6. The electron gun of claim 5 in which the aperture in said second electrode of said second pair of electrodes is larger than the apertures in the other electrodes.

7. The gun of claim 6 in which said lens electrodes comprise two olpposed elliptical members.

8. he gun of claim 7 in which a plurality of tubular ceramic spacers are positioned between adjacent electrodes, a continuous tubular ceramic rod extends through each set of aligned spacers, and a tie rod extends through each ceramic rod. 

1. An electron gun comprising a cathode, first and second planar anodes spaced from said cathode, each of said anodes having a central aperture, a pair of control electrodes positioned between said cathode and said first anode and cooperating therewith to from a circular electron beam which passes through the aperture in said first anode, and means positioned between said anodes for establishing a nonsymmetrical focusing field so that the beam is substantially rectangular in cross section as it passes through the aperture in said second anode.
 2. An electron gun comprising an electron-emitting source, a pair of planar anodes spaced from each other and from said source, a pair of control electrodes positioned between said source a first of said anodes, said anodes having aligned apertures and said control electrodes focusing electrons emitted from said source to form a circular cross section electron beam traversing the aperture in said first anode, and electrode means positioned between said anodes for providing a nonsymmetrical focusing field to change the cross section of the electron beam from a circle as it traverses the aperture in said first anode to substantially a rectangle as it traverses the aperture of the second of said anodes.
 3. The gun of claim 2 in which said electrode means comprises a pair of curved deflecting plates positioned between said anodes for establishing a transverse deflecting field in addition to the said nonsymmetrical focusing field.
 4. The gun of claim 3 in which said plates are opposed elliptical members.
 5. AN electron gun comprising four spaced, parallel, circular electrodes having axially aligned apertures, thermionic cathode means positioned adjacent the aperture in a first of said electrodes, said electrodes formed into two pairs, the first pair of said electrodes comprising the said first electrode and the one adjacent thereto being maintained at a low potential and the second pair of electrodes comprising the remaining two electrodes being maintained at a high positive potential to from an electron beam having a circular cross section as it traverses the aperture in the first electrode of said second pair of electrodes, and a pair of lens electrodes positioned between the second pair of electrodes for establishing a nonsymmetrical focusing field to change the cross section of the electron beam to a rectangular shape when it traverses the aperture in the second electrode of said second pair of electrodes and a transverse deflecting field to deflect the beam at right angles to its major dimension.
 6. The electron gun of claim 5 in which the aperture in said second electrode of said second pair of electrodes is larger than the apertures in the other electrodes.
 7. The gun of claim 6 in which said lens electrodes comprise two opposed elliptical members.
 8. The gun of claim 7 in which a plurality of tubular ceramic spacers are positioned between adjacent electrodes, a continuous tubular ceramic rod extends through each set of aligned spacers, and a tie rod extends through each ceramic rod. 